1. Field of the Invention
The present invention relates to a method and an apparatus for curing adhesive, which are suitable for curing adhesive applied between substrates such as those for optical discs and the like, and an apparatus for bonding disc substrates.
2. Description of Related Art
In general, an optical disc such as a DVD or the like has a structure in which two transparent substrates are bonded together by adhesive. Some of them have a recording layer including a reflective layer or a semi-reflective layer formed on only one of the substrates, and some have recording layers formed on both substrates. In the case where a recording layer is formed on only one substrate, the thickness of the two substrates may be the same, or the substrate on which no recording layer is formed may be formed with a thin, transparent sheet. Some have a structure in which two pairs of substrates are bonded together in a laminate of four substrates. Furthermore, in some cases a plurality of sheets of transparent glass or lenses are bonded to each other via intermediate adhesive.
When manufacturing such an optical disc, after two substrates have been stacked via intermediate adhesive, the adhesive is spread evenly over the substrates by high speed rotation, and excess adhesive is removed. Afterwards, ultraviolet light is normally radiated onto one or both sides of the substrate to cure the adhesive quickly. Regarding the ultraviolet light radiation, ultraviolet light is radiated continuously for a prescribed time using a UV lamp, or pulsed ultraviolet light is radiated using a xenon lamp.
However, the method of curing using these lamps has the following problems.
(1) Since ultraviolet light emitting lamps have low luminous efficiency, and generate significant heat, there is a possibility of the heat distorting the substrates. Furthermore, since a suitable heat dissipation mechanism is required, the size of the apparatus increases, and the cost is high.
(2) Since ultraviolet light emitting lamps are expensive, and their lifetime is short, the running costs are high. Those with a short lifetime must be changed after several tens of hours, which has a detrimental effect on productivity.
(3) In the case of radiating pulsed ultraviolet light, there is an advantage in regard to heat compared with continuous radiation. However, the shock during irradiation is great, so there is a possibility in that vibration during that time may damage the objects to be bonded, such as glass, or have a detrimental effect on adhesiveness. Furthermore, the sound caused by the shocks during irradiation increases the ambient noise level and is not desirable environmentally. In order to solve these problems, conventionally a damping mechanism or a noise control mechanism has been installed. However, this invites a further increase in the size of the apparatus and increases costs.
(4) In the case of lamps, power loss is considerable, so there are disadvantages from both environmental and cost standpoints.
Examples of a conventional DVD manufacturing apparatus and its manufacturing method are described in Japanese Unexamined Patent Application, First Publication No. 2002-245692 (pages 6 to 8, FIG. 1), and Japanese Unexamined Patent Application, First Publication No. 09-231625 (pages 3 to 5, FIG. 1).
In the apparatus described in the references above, adhesive is placed between two disc substrates, the adhesive is spread by a spinner, and the disc substrates are transferred to a pedestal by a transfer mechanism. During this transfer process the adhesive is not cured, and hence there is a possibility in that disc substrates newly bonded together may move. If this happens, they will cure in this state, which causes a drop in quality. Furthermore, there is also a concern that the area around the center hole of the stacked disc substrates may be pulled apart and contaminated by bubbles when the center holes of the disc substrates are mounted on the center pin of a mounting stage in an ultraviolet light irradiating apparatus. Consequently, the uniformity in the tilt and thickness of the completed optical disc is affected, and there is a problem that the quality of the optical disc and the production efficiency deteriorate.
Especially, the uniformity of the tilt and thickness is a major problem for Blu-Ray Disc, which have a very thin, 0.1 mm thick light transmission layer formed from an adhesive layer and a protective sheet, or large capacity optical discs called AOD (Advanced Optical Disc) in which two disc substrates with a thickness of 0.6 mm, which is the same as a DVD, are bonded together, and which require a sufficiently tight tolerance on the thickness of the adhesive film.
In order to solve the above problems, Japanese Unexamined Patent Application, First Publication No. 10-97740 discloses a technique in which disc substrates are transferred after high speed rotation, and the stack of disc substrates is centered, and then cured from above by an emission mechanism moved from a different location, for tacking.
However, this method requires a process in which disc substrates are mounted on a mounting stage for centering, and then a retracted emission mechanism is moved over the disc substrates. Accordingly, operating the apparatus at high speed is difficult, and requires appropriate mechanisms. Furthermore, the inside diameters of the center holes of the disc substrates vary, and hence there is a concern that the inside diameters do not match when the two disc substrates are laminated. Conventionally, when the diameter of a metal member formed from a plurality of blocks is expanded, and applies a pressure to the internal circumferences of the center holes of the disc substrates to center them, a high load is applied to disc substrates with a small inside diameter, which affects the tilt of the disc substrates. Therefore, fine pressure adjustment is difficult.